Laravel Mailgun 401 Unauthorized

I believe I have set up Mailgun quite correctly but am still getting 401 UNAUTHORIZED from Laravel. Here's how I've done it:

In .env:

MAIL_DRIVER=mailgun
MAILGUN_DOMAIN=mg.sample.domain
MAINGUN_SECRET=key-xxxxxxxxxxxxxxxxxxxxxxxx

In config/mail.php:

'driver' => env('MAIL_DRIVER', 'smtp'),

When I send email, I get this error:

Client error: `POST https://api.mailgun.net/v3/mg.sample.domain/messages.mime` resulted in a `401 UNAUTHORIZED` response:
Forbidden

Are my details correct? Yes, because I was able to make it work using the command line:

curl -s --user 'api:key-xxxxxxxxxxxxxxxxxxx' https://api.mailgun.net/v3/mg.sample.domain/messages -F from='Excited User <user@sample.domain>' -F to=user@gmail.com -F subject='Hello' -F text='Testing some Mailgun awesomness!'

{
  "id": "<20170311062047.101018.38081.E6E0ED9A@mg.sample.domain>",
  "message": "Queued. Thank you."
}

One thing to note is the POST url used. The Mailgun docs say we should use /messages, which is what I used from the command line, but Laravel is sending to /messages.mime. Is something wrong here? When I try to POST to /messages.mime from command line, I get:

{
  "message": "'message' parameter is missing"
}

Any idea what's going on?

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